Method of steel processing combining thermal and mechanical surface treatment to control metallurgical phase and mechanical response

ABSTRACT

A method of steel processing combining thermal and mechanical processing of steels in controlled sequences. The method of the present invention combines thermal and mechanical processing in controlled sequences to achieve material property results that are superior to existing methods. The method allows for manipulation of steel processing variables, which promotes further elimination of retained austenite, additional residual compression, reduced surface tension, increased material strength, increased compressive stresses at the surface, and significantly improved bending fatigue and wear resistance. By varying the sequence of mechanical processing of the steel, desired residual compressive stress responses and hardness levels may be achieved. In addition, this processing can reduce embrittlement caused by late stage phase transformation.

CROSS-REFERENCE TO RELATED APPLICATION

This application claims the benefit of priority of U.S. provisionalapplication number 62/649,147, filed Mar. 28, 2018, the contents ofwhich are herein incorporated by reference.

BACKGROUND OF THE INVENTION

The present invention relates to metallurgical processing of alloysteels for control of metallurgical phase, mechanical properties, and inservice part performance. In particular, the patent pertains to thecontrolled sequencing of thermal processing (heat treatment) withintermediate and or post mechanical treatments.

In the heat treatment of steel, quench hardening by itself or incombination with carburizing, nitriding, nitrocarburizng, orcarbonitriding all have limiting factors that prevent optimal hardeningand other desired mechanical responses due to the steels' metallurgy.These factors include, but are not limited to: a) Retained austenitestability; b) carburization response (depth of carbon diffusion); c)formation of intergranular and transgranular carbides; d) residualstress response to the thermal treatment; e) depth of nitiriding; and f)brittleness, hardness of the thermally treated surface layer, amongothers. In particular, tempering practices designed to control finalhardness and strength in the steel may limit service performance interms of bending fatigue and wear resistance. Thus current processingmethods are limited in their ability to affect steel properties. Aspecific example is the inability to affect hardness, strength, residualcompression and bending fatigue life individually. Another example isthe ability to eliminate retained austenite without inducing tensilestresses and cracking.

Existing problems with current methods include an inability to eliminateretained austenite, shallow residual compression, residual surfacetension, and fatigue life limitations in carburized parts from post heattreatment surface processing such as laser peening and other mechanicaltreatments.

The patent defines improved metallurgical processes to help alleviatethese problems.

SUMMARY OF THE INVENTION

In one aspect of the present invention, a method of treating steel isdisclosed. The method includes: heating the steel; applying a quenchhardening sequence to the steel; applying a mechanical surface treatmentto the steel after the quench hardening; and applying one or more cyclesof a deep freeze and a tempering sequence to the steel. Mechanicaltreatments include the group consisting of shot peening, laser peening,low plasticity burnishing, cavitation peening, vibratory processing, andother methods applying kinetic energy to affect internal strain. Thesteel may include a non-carburized steel. The steel may also include oneof a carburized, a nitrocarburized, a carbonitrided, a nitrided, aprecipitation hardened, an induction hardened, a flame hardened, amongother steels.

In a preferred embodiment, the mechanical treatment is a laser peeningprocess. In other embodiments, the mechanical treatment is appliedimmediately following the quench hardening step.

In other aspects of the invention, a method of treating steel includes:heating the steel; applying a quench hardening to the steel to thermallydestabilize retained austenite; applying a first cycle of a deep freezeand a tempering sequence to the steel; applying a mechanical treatmentto the steel; and applying one or more cycles of a subsequent deepfreeze and a subsequent tempering sequence to the steel.

The mechanical treatments include shot peening, laser peening, lowplasticity burnishing, cavitation peening, vibratory processing, andother methods applying kinetic energy to affect internal strain. In apreferred embodiment, the mechanical treatment is a laser peeningprocess. The steel may be a non-carburized steel. The steel may alsoinclude one of a carburized, a nitrocarburized, a carbonitrided, anitrided, a precipitation hardened, an induction hardened, a flamehardened, among other steels. The mechanical treatment may be appliedanytime after the initial quench hardening process.

In yet other aspects of the invention, a method of treating steelincludes: heating the steel; applying quench hardening to the steel;applying a first cycle of a deep freeze and a tempering sequence to thesteel; applying an intermediate mechanical treatment to the steel afterthe first cycle; applying a subsequent cycle of the deep freeze andtempering to the steel, after the intermediate mechanical treatment; andthen applying a second mechanical treatment sequence to the steel.

In some embodiments, the mechanical treatments include shot peening,laser peening, low plasticity burnishing, cavitation peening, vibratoryprocessing, and other methods applying kinetic energy to affect internalstrain. Preferably, the mechanical treatment is applied immediatelyfollowing each subsequent cycle of the deep freeze and the temperingsequence.

The steel may be a non-carburized steel. The steel may also include oneof a carburized, a nitrocarburized, a carbonitrided, a nitrided, aprecipitation hardened, an induction hardened, a flame hardened, amongother steels. These and other features, aspects and advantages of thepresent invention will become better understood with reference to thefollowing drawings, description and claims.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a flow diagram of current standard process sequence;

FIG. 2 is a flow diagram of process sequence A;

FIG. 3 is a flow diagram of process sequence B;

FIG. 4 is a flow diagram of process sequence C;

FIG. 5 are photos of carburized and hardened Ferrium S53 steel subjectedto final laser peening treatment;

FIG. 6 are photos of carburized Ferrium S53 steel subjected to laserpeening treatment prior to tempering; and

FIG. 7 are photos of carburized Ferrium S53 steel subjected to laserpeening treatment after the first deep freeze/temper sequence, butbefore the second deep freeze/temper treatment.

DETAILED DESCRIPTION OF THE INVENTION

The following detailed description is of the best currently contemplatedmodes of carrying out exemplary embodiments of the invention. Thedescription is not to be taken in a limiting sense, but is made merelyfor the purpose of illustrating the general principles of the invention.

Broadly, embodiments of the present invention provide for improvedprocessing techniques for steel, including carburized, anitrocarburized, a carbonitrided, a nitrided, a precipitation hardened,an induction hardened, or a flame hardened, among other steels. Thepresent invention provides a means for application of combined thermaland mechanical processing of heat treated steel in specific sequences,so as to affect one or more of the following beneficial responses to thealloy steel (including but not limited to): 1) Controlled residualcompressive stress depth; 2) Reduction or elimination of retainedaustenite in the heat treated structure; 3) Precipitation and orderingof alloy carbides prior to and during tempering and the mechanicalprocessing; 4) Enhancement of residual compressive stresses in thesoftened condition so as to reduce cracking potential during partservice loading; 5) Ability to thermally refine (temper) the hardenedmicrostructure obtained by the mechanical treatment; and 6) Maintain abeneficial residual compressive stress state through the part crosssection.

The method of the present invention combines thermal and mechanicalprocessing in controlled sequences to achieve material property resultsthat are superior to existing methods. The method allows formanipulation of steel processing variables, which promote furtherelimination of retained austenite, additional residual compression,reduced surface tension, increased material strength, increasedcompressive stresses at the surface, and improved bending fatigue andwear resistance.

The drawings of FIG. 1 show a conventional process of heat treatment andmechanical treatment. Currently, a full steel heat treatment 20 isconducted followed by a mechanical surface treatment 30. This is done inan attempt to achieve improved surface compressive stresses. However,inherent metallurgical complexities in the deep freeze 24 and temperingtreatment 25 cycles of highly alloyed carburized steels 22 produce amicrostructure prone to embrittlement. This embrittlement leads toreduced fracture toughness. The embrittlement and reduced fracturetoughness result from “pockets” of untempered martensite transformingfrom remnants of retained austenite in the carburized case 22. Thistransformation is due to mechanical processing 30 following completedheat treatment 20.

FIG. 5 shows an example of this structure for carburized and hardenedFerrium S53 steel 22 subjected to a final laser peening treatment 32. Itis commonly seen that many carburized, alloy steels 22 subjected to thestandard final laser peening process 32 exhibit a reduction in fatigueresistance due to reduced fracture toughness. This reduction in fatigueresistance and reduced fracture toughness is a direct result of thedescribed mechanically induced martensite transformation. Therefore,alternative sequencing is required to achieve both the microstructuralintegrity and mechanical toughness required in part service. This isespecially true for carburized steel 22 prone to retained austeniteformation. In the standard process 20, after optional carburization,nitrocarburizing, carbonitriding, nitriding, or other treatments, theselected steel 21, 22 first undergoes a quench hardening step 23. Thequench hardening 23 may include gas, oil, water, a salt bath, polymer,etc. The quenched steel may then undergo one or more deep freeze 24 andtempering 25 cycles, followed by a subsequent mechanical processing 30.This mechanical processing 30 may include one or more of a shot peening31, a laser peening 32, a cavitation peening 33, a low plasticityburnishing 34, other mechanical processes 35 including vibratoryprocessing and other methods applying kinetic energy to affect internalstrain.

The best currently contemplated modes of the exemplary embodiments ofthe invention include but are not limited to three possible variationsin thermal / mechanical sequencing.

In a first process of the invention, shown in FIG. 2, the mechanicaltreatment 30 is conducted immediately after the quench hardening step23. This is done to achieve maximum retained austenite transformationprior to the deep freeze 24/tempering 25 sequence. FIG. 6. showscarburized S53's microstructure. This was achieved by a laser peeningprocess 32 prior to tempering 25. This microstructure is a highlytwinned, high carbon martensite consisting of heavy acicular plates. Theinherent retained austenite has been almost fully transformed tomartensite. Hardness prior to tempering 25 here is HRC 48. FIG. 6 alsoshows the final microstructure for carburized Ferrium S53 steel afterheat treatment, then being laser peened 32, followed by a double deepfreeze 24 and tempering 25 treatment. Here the microstructure has ahardness of HRC 58-61. Note the absence of untempered martensitecompared with the structure shown in FIG. 5.

The second embodiment of the invention is the processing sequencereferenced in FIG. 3. In this process, the mechanical processingtreatment 30 is conducted after a first 24, 25, but before the second(and possible subsequent) deep freeze 24′/temper 25′ treatments. This isdone to better maintain residual compressive stress response throughoutthe process, while achieving a higher hardness due to enhanced secondarycarbide precipitation.

The initial deep freeze 24 and temper 25 cycle thermally destabilizesthe retained austenite, which is then more readily transformed to thehighly twinned, acicular plate martensite. This martensite is thentempered out during the remaining thermal sequencing 24′, 25′.Subsequent deep freezing provides additional destabilizing potential tothe remaining retained austenite. The subsequent deep freeze24′/tempering 25′ steps maintain residual compression, but enhancehardness. FIG. 7 shows an example microstructure for carburized FerriumS53 steel achieved by this process. The associated hardness is alsoprovided.

In a third embodiment of the invention, shown in FIG. 4, mechanicalprocessing 30 is conducted after the first 24, 25, second 24′, 25′, andany subsequent deep freeze 24″/temper 25″ sequences. In this processsequence, an intermediate mechanical processing 30 achieves retainedaustenite transformation and deep residual compression. A finalmechanical process 30 is used to locally compress and harden the workingsurface to further enhance wear and bending fatigue performance.

As shown, each of the elements of the steel processing method can bearranged in specific sequences to have specific functional results inthe final steel product. The sequencing is key to achievingmetallurgical response not currently achievable by conventional means.

In Sequence A (FIG. 2), the process seeks to achieve early stageretained austenite transformation, and introduction of residualcompression prior to the deep freeze/tempering sequence designed forsecondary hardening.

In Sequence B (FIG. 3), the process seeks to achieve residualcompression after the initial secondary hardening during whichpreliminary austenite destabilization occurs. In this sequencing,embrittling of the structure is reduced.

In Sequence C (FIG. 4), the process uses a final mechanical treatment30′ to impart local additional increase in residual compression.Non-volumetric shock wave/deformation techniques (i.e. shot peening 31,cavitation peening 34, low plasticity burnishing 33, or other mechanicaltreatments) may be employed in the final stage 30′ to achieve thisobjective. Detrimental retained austenite transformation to martensitenear the surface will not occur in this final step 30′ as it has beencompleted in the sequence prior to tempering 25′.

It should be understood, of course, that the foregoing relates toexemplary embodiments of the invention and that modifications may bemade without departing from the spirit and scope of the invention as setforth in the following claims.

What is claimed is:
 1. A method of treating steel, comprising: heatingthe steel; applying a quench hardening to the steel; applying amechanical treatment to the steel after the quench hardening; andapplying one or more cycles of a deep freeze and a tempering sequence tothe steel.
 2. The method of claim 1, wherein the mechanical treatmentsare selected from the group consisting of shot peening, laser peening,low plasticity burnishing, cavitation peening, and vibratory processing.3. The method of claim 1, wherein the steel comprises: a non-carburizedsteel.
 4. The method of claim 1, wherein the steel comprises:carburized, a nitrocarburized, a carbonitrided, a nitrided, aprecipitation hardened, an induction hardened, and a flame hardened. 5.The method of claim 1, wherein the mechanical treatment is appliedimmediately following the quench hardening step.
 6. A method of treatingsteel, comprising: heating the steel; applying a quench hardening to thesteel to thermally destabilize retained austenite in the steel; applyinga first cycle of a deep freeze and a tempering sequence to the steel;applying a mechanical treatment to the steel; and applying one or morecycles of a subsequent deep freeze and a subsequent tempering sequenceto the steel.
 7. The method of claim 6, wherein the mechanical treatmentis selected from the group consisting of shot peening, laser peening,low plasticity burnishing, cavitation peening, and vibratory processing.8. The method of claim 6, wherein the steel comprises: a non-carburizedsteel.
 9. The method of claim 6, wherein the steel is selected from thegroup consisting of a carburized, a nitrocarburized, a carbonitrided, anitrided, a precipitation hardened, an induction hardened, or a flamehardened steel.
 10. The method of claim 6, wherein the mechanicaltreatment is applied immediately following immediately after the firstcycle.
 11. A method of treating steel, comprising: heating the steel;applying a quench hardening to the steel; applying a first cycle of adeep freeze and a tempering sequence to the steel; applying anintermediate mechanical treatment to the steel after the first cycle;applying a subsequent cycle of the deep freeze and the temperingsequence to the steel, after the intermediate mechanical treatment; andapplying a second mechanical treatment sequence to the steel.
 12. Themethod of claim 11, wherein the mechanical surface treatment is selectedfrom the group consisting of shot peening, laser peening, low plasticityburnishing, cavitation peening, and vibratory processing.
 13. The methodof claim 11, wherein the steel comprises: a non-carburized steel. 14.The method of claim 11, wherein the steel is selected from the groupconsisting of a carburized, a nitrocarburized, a carbonitrided, anitrided, a precipitation hardened, an induction hardened, or a flamehardened steel.
 15. The method of claim 11, wherein the mechanicalsurface treatment is applied immediately following each subsequent cycleof the deep freeze and the tempering sequence.